Fast passage ENDOR techniques were applied to the study of the β-proton hyperfine splittings found in the initial carboxylic anions created in irradiated single crystals of succinic acid and thiodiglycolic acid. The associated tensors for each acid were found to be very similar and indicated both were essentially unchanged from the undamaged molecular conformation. For thiodiglycolic acid the hyperfine tensor associated with the polar proton was also determined and shown to be based primarily on hyperconjugation.